1. Field of the invention
This invention relates to a robot hand for stacking boxes in which lower ends of forks for supporting flanges on upper ends of a box are set at a predetermined level difference.
2. Description of the Prior Art
In these days, robots are widely used as labor-saving means for transferring, stacking and disassembling freight or goods which enhances the efficiency of loading and unloading operations. Generally, forks 1 and 2 opposed to each other of the robot hand have their positions for gripping a box 3 in the same level l--l line as shown in FIG. 2. Therefore, when freight, particularly boxes 3, having a projected portion a at the lower end thereof as shown, are successively stacked vertically by fitting the projected portion a, into an open position b at the upper end of a box 4 directly therebelow, and then the forks 1 and 2 are simultaneously spread to drop the box 3. However, the forks 1 and 2 are not always spread open simultaneously through the same angle. In case of material in the box 3, for example, powder or granular material g such as cement, sugar and cereals, the box may tilt and the projected portion a dropped in an inclined state onto the open portion b after the forks 1 and 2 have been spread open due to the variation of the position of the center of gravity of the material. Furthermore, there is a possibility that the box 4 undergoes vibrations and shocks due to the weight of the material g within the box 3 and fails to be held in a given position, and therefore, the suspending and gripping position of the box 3 by the forks 1 and 2 cannot be set at a given position and the position of the forks 1 and 2 unavoidably needs to be adjusted every time the operation takes place. As a consequence, this leads to one main factor which impairs the enhancement of working efficiency.
Moreover, there is a further possibility that the box 4 and material w therein which is a solid will be damaged and cracked due to the weight of material within the box 3 to be dropped. Accordingly, the stacking operation of the boxes 3 and 4 by the simultaneous spreading of the forks 1 and 2 at the same level is not always performed vertically and orderly. Sometimes considerable labor and time are required for adjustment.